全文获取类型
收费全文 | 5281篇 |
免费 | 230篇 |
国内免费 | 8篇 |
专业分类
林业 | 314篇 |
农学 | 160篇 |
基础科学 | 44篇 |
795篇 | |
综合类 | 836篇 |
农作物 | 242篇 |
水产渔业 | 453篇 |
畜牧兽医 | 2095篇 |
园艺 | 153篇 |
植物保护 | 427篇 |
出版年
2023年 | 34篇 |
2022年 | 40篇 |
2021年 | 100篇 |
2020年 | 123篇 |
2019年 | 128篇 |
2018年 | 88篇 |
2017年 | 106篇 |
2016年 | 102篇 |
2015年 | 110篇 |
2014年 | 123篇 |
2013年 | 248篇 |
2012年 | 291篇 |
2011年 | 356篇 |
2010年 | 195篇 |
2009年 | 156篇 |
2008年 | 348篇 |
2007年 | 295篇 |
2006年 | 300篇 |
2005年 | 303篇 |
2004年 | 269篇 |
2003年 | 279篇 |
2002年 | 264篇 |
2001年 | 60篇 |
2000年 | 68篇 |
1999年 | 73篇 |
1998年 | 52篇 |
1997年 | 49篇 |
1996年 | 43篇 |
1995年 | 38篇 |
1994年 | 47篇 |
1993年 | 54篇 |
1992年 | 46篇 |
1991年 | 34篇 |
1990年 | 41篇 |
1989年 | 35篇 |
1988年 | 32篇 |
1987年 | 37篇 |
1986年 | 28篇 |
1985年 | 36篇 |
1984年 | 41篇 |
1983年 | 38篇 |
1982年 | 20篇 |
1981年 | 33篇 |
1980年 | 23篇 |
1979年 | 24篇 |
1978年 | 22篇 |
1977年 | 18篇 |
1975年 | 18篇 |
1974年 | 23篇 |
1973年 | 24篇 |
排序方式: 共有5519条查询结果,搜索用时 31 毫秒
1.
Lampreys have a complex life cycle which includes a multi‐year infaunal larval stage (ammocoete). Gut content analysis has generally identified detritus (i.e., unidentifiable organic matter) as the major dietary component to ammocoetes, though algae can also be important. However, gut content preserves only a snapshot of the animal's diet and does not reflect assimilated material. In order to better characterise the nutritional sources supporting ammocoete growth, we analysed ammocoete body tissue and potential dietary sources at two streams using natural Δ14C and δ15N to estimate time‐integrated nutritional support. Bayesian isotope mixing models revealed differences in the importance of sources supporting ammocoetes between sites. Ammocoetes from a stream in a mixed land usage area (~50% agriculture, ~40% forest and ~10% developed) were primarily supported (mean: ~50%) by fresh terrestrial organic matter but were also supported by substantial contributions (mean: ~30%) by aged organic matter (AOM) and autochthonous material (algae; mean ~20%). In a predominantly forested (~90%) headwater stream, different modelling scenarios (uninformed or informed priors) suggested that algal support of ammocoete nutrition ranged from 7% to 45%. However, the model relying on informed priors developed from gut content analysis produced the low estimates, suggesting these were more reliable. When algae were a minor component of the nutrition at the forested site, ammocoetes were highly dependent on AOM (83 ± 26%; mean ± SD). Based on these findings, ammocoete growth and development are predicted to be strongly influenced by both land use and the availability of allochthonous and autochthonous materials of varying ages within streams. 相似文献
2.
Although abalone fisheries in South Africa have existed since 1949, cultivation started by successful spawning of captured specimens in 1981 to produce spat and juveniles. Twelve abalone farms, with an estimated investment of US$12 million, have since been established on the coast of South Africa, with a projected production of 500–800 tons. While Haliotis midae could reach a maximum size of about 200 mm shell length at an age of over 30 years in the wild, farm production is concentrating on an average size of 100 mm after 5 years. Growth rates of 0.08–4.5% body weight day ?1 for abalone of 10–17 mm shell length have been found under stimulated farming conditions on formulated diets, while the corresponding feed conversion ratio (FCR) was 0.9–2.4. Growth rate, FCR and protein efficiency ratio decline at water temperatures above 20 °C. Anaesthetics, for example magnesium sulphate, are used to prevent injury during removal from tanks. Prostrate diatoms, such as Cocconeis sublittoralis (Hendey), Amphora proteoides (Hustedt) and Achnanthes brevipes (Agardh) are preferred after the larval stage (5–7 days, depending on temperature) are finished. South Africa abalone growers prefer formulated feeds in pellet form, which is convenience and cost favourable to farm operations and management. Although fish meal has been found to be the most suitable protein source for inclusion in formulated diets, plant proteins such as soy bean meal, cottonseed meal and sunflower meal presented good growth and apparent protein digestibility. The parasite sabellid polychaete, Terebrasabella heterouncinata, recently named by Australian and American researchers, is indigenous to a variety of South African intertidal molluscs and impedes growth by causing irritation beneath the mantle in abalone. Prevention to some degree is possible by high standards of hygiene and husbandry of abalone in the tanks, but new techniques to control large infestations now include ultrasound. In spite of a reputation for toughness, abalone meat frozen immediately after shucking is tender relative to red meat. The success of abalone farming in South Africa has been as a result of a high degree of cooperation between the private sector and government‐backed research institutions, and it is anticipated that this collaboration will continue. 相似文献
3.
4.
Jeffrey R. Crass MD Ronald L. Genovese VMD James A. Render DVM Errol M. Bellon MD 《Veterinary radiology & ultrasound》1992,33(4):206-216
Ultrasonography and MRI have become valuable tools for imaging of tendon injuries. The current study examines the histopathologic basis for the imaging abnormalities. Five injured equine forelimbs and two normal contralateral limbs were studied with high resolution real time ultrasound and MRI. Histologic sections were made and correlated with the diagnostic images. All lesions were readily seen by both modalities. Lesions characterized by hemorrhage, edma, and cellular infiltration were sonolucent on ultrasound and bright on MRI images. MRI returned to normal as fibrogenesis ensued. Ultrasound images remained abnormal until fibrillar reallgnment occured with completion of the healing process. High resolution real time ultrasound and MRI both accurately reflect the tissue abnormalities in acute tendon injuries. The injured horse does appear to be an effective model for correlative imaging studies of tendon injuries. 相似文献
5.
James Mintert 《Veterinary Clinics of North America: Food Animal Practice》2003,19(2):387-395
The United States cattle feeding sector has changed dramatically over time. Movement from an industry largely populated by farmer feeders using livestock to market their grain production to an industry composed of large, specialized, commercial cattle feeding firms has occurred since the end of World War II. Moreover, the industry also shifted geographically, moving away from the Corn Belt to the Plains as farmer feeders exited the industry. Today, the 10 largest firms in the industry likely market 22% to as much as 29% of all fed cattle produced in the United States. The shift in industry structure was at least partially motivated by the advent of improved technology. Successful implementation of rapidly evolving technology often required more intense, specialized management (and in some cases, larger operations) than was available in many farmer feeder operations. The ongoing shift away from live-weight pricing of fed cattle toward grid-based pricing could increase the need for specialized management yet again, encouraging even more industry consolidation in the future. 相似文献
6.
7.
A stripspawning methodology was evaluated for tilapia (oreochromid) species. This technique achieved an average hatching success of 68.6 ± 3.6% (N= 31). Female fecundity and spawning frequency were dependant on both genetical and husbandry factors. Egg yields for Oreochromis niloticus, O. mossambicus, and O. niloticus±O. mossambicus hybrids averaged 4.54, 10.86 and 10.36 eggs/g female/spawn, respectively. Female broodstock that were adapted to an intensive spawning regime exhibited a significant increase in fecundity. Additionally, egg survival was not affected by hydration for up to 15 minutes prior to fertilization. Results suggest that the strip spawning of tilapia species may be an efficient method of providing viable gametes for hatchery purposes. 相似文献
8.
Ronald D. Hodges DVM Russell L. Tucker DVM James J. Brace DVM 《Veterinary radiology & ultrasound》1993,34(4):249-252
9.
David A. Detweiler David S. Biller DVM James J. Hoskinson DVM Kenneth R. Harkin DVM 《Veterinary radiology & ultrasound》2001,42(2):108-112
Canine dysautonomia is an idiopathic condition resulting in loss of autonomic nervous system function. Recently, the prevalence of dogs diagnosed with dysautonomia in the mid-western United States has increased. In this study the medical records and radiographic findings in 24 dogs with dysautonomia were reviewed. A diagnosis of dysautonomia was made in 17 (71%) of the dogs in this study by postmortem examination, the remaining 7 (29%) dogs were diagnosed pharmacologically. The radiographic findings supportive of dysautonomia include aspiration pneumonia, megaesophagus, or a distended stomach, small bowel, or urinary bladder. In some instances, the disease radiographically mimicked other disorders of the gastrointestinal tract, including mechanical obstruction. 相似文献
10.